1. Field
The present teachings generally relate to the field of imaging using waves and more particularly, to systems and methods for improving the resolution of images obtained by an array of transmitters and receivers.
2. Description of the Related Art
Imaging devices such as ultrasound devices transmit a signal into a medium, and measure reflections from various features in the medium. An image of a given feature can be reconstructed based on quantities such as intensity and frequency of the reflected signal from that feature. To form the image, one needs to somehow determine the relative location of the feature with respect to the imaging device, and in particular, to a location of a receiving element of the device.
Conventional ultrasound devices typically have an array of transmitter-receiver pairs. In operation, each pair only “sees” along a line, commonly referred to as a scanline, that extends from the pair into the medium. With such an assumption, a feature along that scanline can be brought into “focus” by determining the propagation times of the transmitted and reflected signals to and from the feature. A propagation time t can be calculated as t=d/v where v is the velocity of the signal in the medium, and d is the distance of interest (e.g., from the feature to the receiver). The distance d can be determined by dividing the scanline into discrete elements in a predetermined manner, such that the location of each element is known. The velocity v can either be assumed as a constant in the medium, or can be calculated in a manner generally known in the art.
Based on such an operation, one can see that the resolution and quality of the image formed can be limited by the size of the scanline element. Even if the scanline element can be made arbitrarily small, the effective operation of obtaining an image from the device is subject to the intrinsic resolution of the transmitter-receiver pair, as well as the sampling criteria for yielding a meaningful result.
The intrinsic resolution of a detector can be expressed as depending on the ratio of the operating wavelength of the signal to the effective dimension of the detector (commonly referred to as a Rayleigh or Sparrow criteria). One can change such a resolution of the detector by either changing the wavelength and/or the effective dimension of the detector. For example, reducing the wavelength (increasing the frequency) and/or increasing the effective dimension of the detector can improve the resolution. However, such a change can be accompanied by undesired effects. For example, an increased frequency signal has a smaller penetration depth in ultrasound applications.
Furthermore, an increased operating frequency also forces the minimum sampling frequency. Commonly known as the Nyquist sampling criteria, a signal needs to be sampled at a frequency that is at least approximately twice the frequency of the operating signal to yield a meaningful result.
Because of the foregoing challenges and limitations, there is an ongoing need to improve the manner in which imaging devices are designed and operated.